1. Field of the Invention
This invention relates to improved articles produced from modified, impact resistant polystyrene compositions.
2. Description of the Prior Art
Polystyrene is an economically priced thermoplastic material having many desirable properties. For a variety of uses polystyrene is employed as an impact-improved product, made as a graft polymer of styrene and an elastomer, typically a butadiene-containing polymer, particularly polybutadiene.
An undesirable property of polystyrene is its tendency to deteriorate drastically under the influence of materials which induce stress-cracking, such as fatty substances, e.g., fatty foodstuffs. The deterioration consists of surface crazing of polystyrene articles and drastic loss of impact strength of articles made from impact-improved polystyrene compositions. Some major fields of use have been foreclosed to polystyrene because of these objectionable properties. Styrene copolymers such as ABS (a copolymer of acrylonitrile, butadiene and styrene) are used where resistance to chemically induced stress cracking is required, such as in containers for fatty foodstuffs.
The present invention overcomes these deficiencies of conventional high impact polystyrene and provides articles, made from a relatively inexpensive composition based on polystyrene-diolefin graft copolymers and polyolefins, which are substantially a resistant to chemically induced stress cracking, such as deterioration by fatty substances, as articles made from ABS, and typically have the impact resistance of impact-improved polystyrene.
It is known that blends of polystyrene and polyolefins generally have very poor impact properties because the polymers tend to form separate domains within an article, so that articles formed from such blends delaminate under stress.
It has been taught that certain materials, and particularly certain block copolymers, can be employed to improve the compatibility of styrenic and olefinic polymers in blends. Such disclosures are contained in two U.S. patents to Agouri et al. and in a series of patent cases of Badische Anilin und Soda Fabrik A.G. (BASF).
U.S. Pat. No. 3,894,117 to Agouri et al., is directed to the preparation of "homogeneous" polymer compositions containing at least one olefin polymer and/or one arylvinyl polymer. The method consists of adding to either or to a mixture of both of these polymers one or more block copolymers containing one polyolefin sequence and one polyarylvinyl sequence. The patent discloses compositions directed to various different types of uses. Thus, papers substitutes of excellent quality for printing work can be obtained from composites of 5 to 30 wt.% of the block copolymers, 2-4% polystyrene and the rest polyethylene. Composites with higher block copolymer content, and particularly low density polyethylene containing 20-50 wt.% block copolymers, are said to be useful for making articles such as bottles and pipes. Using extremely low concentrations of the block copolymers in polyolefins or polyolefin copolymers such as ethylene/vinylacetate copolymers is said to reduce post extrusion blocking. In all cases, the block copolymer used contains only two blocks.
U.S. Pat. No. 3,980,736 to Agouri et al. is a division of the above discussed patent, with an apparently identical specification.
A series of patents assigned to BASF illustrate gradual evolution of concepts dealing with the use of certain block copolymers. They will be briefly discussed in the order of their premier depot filing dates, which are stated in the parentheses following the patent number.
U.S. Pat. No. 4,031,166 (1/29/1970) is directed to substituting for the conventional elastomer in impact-improved polystyrene elastomer blends a mixture of a polyolefin and a substantially gel-free vinyl aromatic/diene hydrocarbon polymer which may be a block copolymer, a random copolymer, or a graft copolymer. It is preferred to cross-link the resulting product. The experimental examples illustrate the invention with blends of polystyrene with low density polyethylene and the following copolymers: a styrene-butadiene-styrene block copolymer X-Y-X in which the molecular weight of the polystyrene block is an unspecified value in the range 4,000 to 215,000 and that of the polybutadiene block in the range 20,000 to 250,000; a similar X-Y-X copolymer of 25% styrene content and unstated molecular weight; a 50:50 styrene-butadiene block copolymer of unstated molecular weight; a 50:50 styrene-hydrogenated butadiene block copolymer of unstated molecular weight; a copolymer of 200,000 molecular weight metallated polybutadiene, graft-polymerized with an equal amount of styrene; and a random copolymer of butadiene/styrene. It is asserted that use of mixtures of polyolefins with any of these vinyl aromatic/diolefin copolymers in lieu of conventional elastomers, in impact polystyrene produced by blending, permits one to obtain a desired impact strength at lower elastomer content and, hence, increased rigidity; it is said to permit production of compositions of improved gloss compared with graft-polymerized impact polystyrene.
U.S. Pat. No. 3,738,907 (corresponding to German printed application 2,029,028, filed 6/12/1970) is directed to laminates of two thermplastics, bonded by an adhesive block copolymer A-B in which A is a polyvinyl aromatic block of 10,000-150,000 molecular weight and B a polyolefin block of 10,000-100,000 molecular weight in which not more than 10% of the carbon-carbon bonds are olefinically unsaturated and the molar ratio A:B is from 10:90 to 70:30, preferably from 30:70 to 60:40. It is said that these two-block copolymers provide a much better bond between the plastics material than prior art three-block copolymers and in addition are simpler to produce than three-block copolymers. The prior art three-block copolymer specifically referred to is a fully saturated A-B-A block described in German printed application number 1,569,423.
U.S. Pat. No. 3,767,521 (10/7/1970)is directed to an improvement in metal-plated sandwich panels made up of a structural thermoplastic inner ply and thin outer metal plies. The inner ply is preferably polystyrene or impact-improved polystyrene, but may be a styrene copolymer or polyethylene, polypropylene or ethylene-propylene copolymer. The improvement consists in using two separate layers of materials to provide improved adhesion between the plastic inner ply and the metal outer ply. The layer next to the metal is a conventional hot-melt adhesive. The layer between the thermoplastic composition and the hot-melt adhesive is designated an "adhesion promoter"; it is a ternary mixture of a homopolymer of styrene or a styrene copolymer with up to 35% of a second copolymer, a homopolymer of a C.sub.2 to C.sub.4 mono-olefin or a copolymer of such a mono-olefin with up to 35% of another monomer, and a two-block copolymer A-B in which A is a block of polystyrene of 10,000-150,000 molecular weight and B is a block of a polyolefin of 10,000-200,000 molecular weight which is at least 90% saturated. The polystyrene-to-polyolefin ratio in the adhesion promoting layer is from 0.25:1 to 4:1 by weight and the percent of block copolymer from 5-70% by weight based on the total blend composition.
United Kingdom patent specification No. 1,357,950 (10/7/1970) is directed to a laminate of two or more different thermoplastic plies including at least one composite ply made up of a thermoplastic mixture comprising (a) a homopolymer or copolymer of a vinylaromatic monomer, (b) a homopolymer or copolymer of an olefin monomer, and (c) a block copolymer A-B, which is as described in U.S. Pat. No. 3,738,907, except that the upper limit of the molecular weight of the polyolefin block is 200,000 instead of 100,000. The ratio of polyvinyl aromatic to polyolefin in this composition is from 0.25:1 to 4:1, and the total content of A-B is between 5 and 70% by weight. The composite ply is employed as an adhesive between two other thermoplastic plies which would otherwise adhere poorly. It is disclosed to be particularly suitable to provide adhesion between impact-resistant polystyrene and low density polyethylene.
U.K. patent specification No. 1,363,466 (12/12/1970) is directed to molding compositions in which a homopolymer of styrene or other vinylbenzene or a copolymer thereof is compounded with a homopolymer of an alpha monoolefinic acyclic hydrocarbon or copolymer thereof, and in which compatibility of these components is improved by the incorporation of a block copolymer A-B having the properties described in the above U.K. Pat. No. 1,357,950.
U.K. Pat. No. 1,363,466 also contains a disclosure that hydrogenated two block copolymers A-B facilitate said mixing much better than similar block copolymers, for example, non-hydrogenated two block copolymers A-B or non-hydrogenated or hydrogenated three and four block copolymers.
German printed application No. 2,137,274 (7/26/1971) is a patent of addition to German application No. 2,003,916, which corresponds to U.S. Pat. No. 4,031,166 supra. It discloses the use of copolymers of ethylene and polar vinyl components, such as ethylene-vinyl acetate, in lieu of the polyolefins of the main patent.
German printed application No. 2,201,243 (1/12/1972) is a patent of addition to German patent application No. 2,061,361. The main patent application is the equivalent of U.K. Pat. No. 1,363,466. In the main German patent, the proportion of block copolymer comprises 10-70 wt.% of the total mass. According to the patent of addition, useful properties are obtained in molding compositions containing 1-10% by weight of block copolymer A-B. It is said that such blends are particularly suitable for production of films according to the blown bubble method. The films have good surface properties and are particularly suitable as adhesives between styrene and olefin polymers for the production of multiple ply composites.
U.K. specification No. 1,427,959 (7/1/1972) is directed to a process for the manufacture of a laminate comprising an adhesion-promoting primer layer between two different thermoplastics, wherein waste material produced in further processing of the laminates is comminuted, mixed, and added during extrusion either to the adhesion promoting primer layer or to one of the outer plies. Block copolymers (X-Y)n or X(Y-X)n where X is a vinyl aromatic polymer, Y is an optionally hydrogenated diolefin polymer and "n" is an integer from 1 to 4, and wherein the transitions between the blocks may be sudden or gradual, are disclosed as the material for the primer layer. Preferred are those block copolymers in which X is a polystyrene block and Y a polybutadiene block. The molecular weight of the block copolymer is generally from 10,000 to 1,000,000. The molar ratio X:Y is from 10:90 to 70:30. The thermoplastics which are bonded by the primer layer may be "virtually any of the well-known plastics"; bonded laminates of an olefin polymer and a styrene polymer are, however, preferred.
German printed application No. 2,236,903 (7/27/1972) is directed to laminates comprising outer layers containing two different thermoplastics which may be any known thermoplastic, bonded by an adhesion-promoting layer which is a mixture of at least 5% of each of the plastics represented in the outer layers and at least 30% of a block copolymer (X-Y)n or X-(X-Y)n where n is 1 to 3 and where X is a polystyrene block and Y a diene polymer block. Block polymers of 2,3, or 4 blocks are preferred. The transition between blocks may be sudden or gradual.
Belgian Pat. No. 814,711 (5/8/1973) is directed to the preparation of foamed materials for insulating buildings, the compositions being prepared from mixtures of polyolefins with 10-95 wt.% styrene polymer, mixed with a volatile blowing agent and 0.5-10 wt.% of a hydrogenated styrene-butadiene block copolymer or a polyolefin grafted with styrene. The block copolymers are not further detailed but reference is made for their preparation to HOUBEN-WEYL, Methoden Der Organischen Chemie, Vol. 14/1, p. 659 (1961).
U.S. Pat. No. 4,020,025 (5/8/1973) is based on the same German application as Belgian Pat. No. 814,711 but has some additional description of the block copolymers, giving the molecular weight of the hydrogenated styrene/butadiene block polymer as 3,000 to 800,000, preferably 10,000 to 100,000, with 10 to 80% styrene and consisting of individual blocks of pure polybutadiene and polystyrene segments or polybutadiene segments containing styrene with blocks of pure polystyrene segments. The patent emphasizes that in the expanded compositions the styrene polymer is dispersed in the polyolefin and the particle size of the styrene polymers is less than 50 and preferably less than 10 microns, the polyolefin forming the continuous phase.
Belgian Pat. No. 826,831 (3-20-1974) is directed to foamable plastic compositions containing a mixture of polystyrene, polyethylene, a combining agent and a blowing agent. A combining agent is a material which when mixed with polystyrene and polyethylene causes a fine distribution of one polymer in the other and may be a graft polymer of styrene on polyethylene or a copolymer of ethylene and styrene. Reference is made to polymers of blocks of styrene and hydrogenated butadiene described in the Houben-Weyl reference supra, page 833. The polymers have molecular weights between 3,000 and 800,000 and contain between 10 and 80% by weight styrene.
In summary, the prior art represented by the Agouri patents shows compositions containing a major proportion of polyolefin, a small proportion of polystyrene and a two-block polystyrene-polyolefin copolymer to improve their compatibility, primarily for the purpose of improving the impact characteristics of the polymer and also to produce a paper substitute.
In the series of BASF patent cases, the first-filed case is broadly directed to the use of a variety of different vinyl aromatic/diene copolymers in admixture with a polyolefin as additives blended with styrene homopolymer to provide impact-improved polystyrene which is said to be superior in rubber utilization to conventional polystyrene/elastomer blends and superior in gloss to graft polymerized high impact polystyrene. The next five BASF patent cases all are directed to using two-block copolymers of polystyrene and hydrogenated diolefin, primarily as adhesion promoters for laminates and also to provide compatibility of polystyrene and polyolefins. In the last-filed four BASF patent cases, use of block copolymers is broadly disclosed, again for the purpose of providing adhesion-promoting compositions, except in the last-filed Belgian and U.S. patents, which are directed to foam compositions in which polystyrene is dispersed as discrete particles in the polyolefin.